Pyrolytic cracking of hydrocarbons is a petrochemical process that is widely used to produce olefins such as ethylene, propylene, butylenes, butadiene, and aromatics such as benzene, toluene, and xylene. The starting feedstock for a conventional olefin production plant is typically subjected to substantial (and expensive) processing before it reaches the olefin plant. For instance, normally, whole crude is first subjected to desalting prior to being distilled or otherwise fractionated into a plurality of parts (fractions) such as gasoline, kerosene, naphtha, atmospheric gas oils, vacuum gas oils (VGO) and pitch, (also called “short resid” or “short residue” or “Vacuum Tower Bottom”). As an alternate to the production of vacuum gas oils and pitch, sometimes a combination of these (usually given the name “long resid” or “long residue”) is produced. The short resid cut typically has a boiling range that begins at a temperature greater than 1050° F. (566° C.), at atmospheric pressure. After removal of the short resid fraction from crude oil or long resid, conventionally, any of their fractions or combinations of them may be passed to a steam cracker as the feedstock. Alternatively, whole crude, after desalting and removal of the “short resid” can also be used as a feedstock.
Conventional steam cracking processes to produce olefins utilize a pyrolysis furnace that generally has two main sections: a convection section and a radiant section. In the conventional pyrolysis furnace, the hydrocarbon feedstock enters the convection section of the furnace as a liquid (except for light feedstocks such as ethane and propane which enter as a vapor) wherein it is heated and vaporized by indirect contact with hot flue gas from the radiant section of the furnace and optionally by direct contact with steam. The feedstock is normally mixed with steam and the feedstock/steam mixture is then introduced through crossover piping into the radiant section where it is quickly heated, at pressures typically ranging from about 10 to about 30 psig, to typical pyrolysis temperatures such as in the range of from about 1450° F. (788° C.) to about 1562° F. (850° C.), to provide thorough pyrolytic cracking of the feed stream. The resulting olefin rich pyrolysis products leave the furnace for further downstream separation and processing.
A recent advance in pyrolysis of crude oil and crude oil fractions containing pitch is shown in U.S. Pat. No. 6,632,351. In the '351 process a crude oil feedstock or crude oil fraction(s) containing pitch is fed, after desalting, directly into a pyrolysis furnace. The process comprises feeding the crude oil or crude oil fractions containing pitch to a first stage preheater within the convection section, where the crude oil or crude oil fractions containing pitch are heated within the first stage preheater to an exit temperature of at least 375° C. to produce a heated gas-liquid mixture. The mixture is withdrawn from the first stage preheater, steam is added and the gas-liquid mixture is fed to a vapor/liquid separator, followed by separating and removing the gas from the liquid in the vapor/liquid separator, and feeding the removed gas to a second preheater provided in the convection zone. The preheated gas is then introduced into a radiant zone within the pyrolysis furnace, and pyrolyzed to olefins and associated by-products. While this is an improvement in the overall process, there are still limitations in achieving higher yields of more valuable products, particularly from the lighter fraction of the vaporized feed. These limitations are due to the conversion to olefins being limited by the milder pyrolysis conditions required to prevent rapid coke formation from pyrolysis of the heavy fraction, either in the pyrolysis coils and/or in the downstream quench exchangers.
U.S. Pat. No. 6,979,757 discloses a process utilizing whole crude oil as a feedstock for the pyrolysis furnace of an olefin production plant wherein the feedstock after preheating is subjected to mild thermal cracking assisted with controlled cavitation conditions until substantially vaporized, the vapors being subjected to severe cracking in the radiant section of the furnace. This process is similarly limited as in the '351 patent as the entire vapor stream is subjected to one pyrolysis severity.
U.S. Pat. No. 4,264,432 discloses a process and system for vaporizing heavy gas oil prior to thermal cracking to olefins, by flashing with steam in a first mixer, superheating the vapor, and flashing in a second mixer the liquid from the first mixer. Such a process is primarily directed to minimizing the amount of dilution steam required for vaporization of heavy gas oils having an end point of about 1005° F. (541° C.) prior to pyrolysis cracking of the heavy oil, and is not directed to creating an acceptable pyrolysis feedstock from an otherwise unacceptable feedstock having undesirable coke precursors and/or high boiling pitch fractions. Again this process is limited as in the '351 and '432 patents described above since the entire vaporized feedstock is cracked at one pyrolysis severity.
U.S. Pat. No. 3,617,493 discloses a process for steam cracking a crude oil feed by first passing it through the convection of a first steam cracking furnace, then separating out in a flash drum separator a vaporized fraction (naphtha and lighter components fraction), and a liquid fraction. The naphtha and lighter fraction is then pyrolyzed in the first cracking furnace. The liquid separated from the flash drum separator is withdrawn and fed to the convection section of a second steam cracking furnace, and thereafter into a second flash drum separator; the vapor from this second separator is then pyrolyzed in a second steam cracking furnace. The use of two separate steam cracking furnaces allows the lighter fraction and the heavier fraction of the crude oil feed to be cracked under different cracking conditions to optimize yields. However, the use of two separate cracking furnaces can be a very costly process choice. Moreover, the process claimed in the '493 patent cannot be easily changed to accommodate changing feed compositions.
U.S. Pat. No. 4,612,795 discloses a process and system for the production of olefins from heavy hydrocarbon feedstocks, by first pretreating the hydrocarbon at high pressure and moderate temperatures to preferentially remove coke precursors. The pretreated hydrocarbon is then separated into a lighter and a heavier fraction in a conventional fractionation column. The lighter and heavier fractions are fed to a pyrolysis furnace having two separate radiant cells. The lighter fraction is cracked in one radiant cell and the heavier fraction in cracked in the other radiant cell thus allowing the two fractions to be cracked separately at their optimal cracking conditions. The heavy bottom product from the fractionation column is used as fuel oil. While U.S. Pat. No. 3,617,493 and U.S. Pat. No. 4,612,795 teach the benefits of separately cracking fractions of wide boiling feedstocks at pyrolysis conditions appropriate for those fractions, they require additional equipment beyond one pyrolysis furnace and are only applied to feedstocks with undesirable heavy feedstock components such as pitch.
It is further known that state-of-the-art pyrolysis furnaces having two separate feedstocks are currently built by pyrolysis furnace designers such as the Stone and Webster division of Shaw Industries. Further details of pyrolysis furnaces with one and two radiant cells cracking two feedstocks simultaneously at optimum cracking conditions are revealed in the article: “Large ethylene furnaces: changing the paradigm” by John R. Brewer of the Stone and Webster Corporation, (published in the ePTQ magazine, 2nd Quarter issue of 2000, pages 111-116). However, in such designs the two feedstocks that are simultaneously fed to the furnaces are already separated, i.e. they are not fed to the furnace as a single wide boiling range feedstock.
The prior art cited above does not teach how to efficiently separate and pyrolyze the various fractions in a wide boiling feedstock to obtain the highest potential yield of olefins using only one steam cracking furnace with one feedstock. What is needed is an improved process that permits the economical processing of a hydrocarbon feedstock having a wide boiling range to produce lower olefins in higher yield by separately cracking the various fractions at the optimal conditions for those fractions in one furnace.